<p>A framework for process monitoring and diagnosis, called Mimic, is described. Mimic is based on the observation that the key cognitive skill for process operators is the formation of a mental model that not only accounts for current observations but also lets them predict near-term behavior as well as the effect of possible control actions. Mimic exploits three relatively new technologies: semiquantitative simulation, measurement interpretation (tracking), and model-based diagnosis. These technologies work together in a hypothesize-build-simulate-match cycle. Each of these technologies is discussed. To illustrate Mimic at work, an electric water heater modeled and tested with Mimic is considered. The advantages and limitations of Mimic, as seen by this example, are examined. Related work is discussed.</p>